Symptoms of abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid rhythm being referred to as tachycardia. The present invention is concerned with the treatment of tachycardias that are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of the chambers of a heart. The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper to the lower chambers necessary for performing normal systole and diastole function. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the electrode or transducer to the cardiac tissue in order to ablate the tissue of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated. However, in the case of atrial fibrillation (AFib), multiple arrhythmogenic sites and/or multiple accessory pathways exist. The conventional catheter with a single ablation tip electrode can not effectively cure the symptoms. In the paroxysmal AFib, the origin of the arrhythmia may lie near the ostium of or inside a pulmonary vein. A complete circular lesion is required around the ostium or the wall of the pulmonary vein to cure the arrhythmia. It is imperative to have an accurate locating system.
Atrial fibrillation is believed to be the result of the simultaneous occurrence of multiple wavelets of functional re-entry of electrical impulses within the atria, resulting in a condition in which the transmission of electrical activity becomes so disorganized that the atria contracts irregularly. Once considered a benign disorder, AFib now is widely recognized as the cause of significant morbidity and mortality. The most dangerous outcome from AFib is thromboembolism and stroke risk, the latter due to the chaotic contractions of the atria causing blood to pool. This in turn can lead to clot formation and the potential for an embolic stroke. According to data from the American Heart Association, about 75,000 strokes per year are AFib-related.
A catheter utilized in the endocardial RF ablation is inserted into a major vein or artery, usually in the neck or groin area. For paroxysmal AFib indications, a catheter is approached from the atrium to the ostium of a pulmonary vein. The tip section of a locator catheter is referred to here as the portion of that catheter shaft containing the electrode means which may be deflectable and may be adapted to form a circular or an irregular-shape complete loop lesion. The electrode means is to be positioned against the ostium of the pulmonary vein or inside the vein, whereby the circular electrode means having a firm wire or coil electrode for circular lesion ablation.
The mapping and ablation procedures require means to locate the catheter; especially the tip section of said catheter, to the exact site of the arrhythmogenic sources. In the case of paroxysmal AFib, the location is about the pulmonary veins. The conventional method uses x-ray fluoroscope to image the location of the catheter. While x-ray imaging is quite successful, some patients, such as the pregnant women, the fluoro-phobic patients and the like, can tolerate little x-ray exposure. It is imperative that other imaging means be used to locate the catheter within the body of a patient.
Ultrasound imaging has been used extensively to reveal the existence of a device having the ultrasound emitter. In the U.S. Pat. No. 4,794,931, there has been disclosed a catheter and system which can be utilized for ultrasonic imaging. However, there is no disclosure on the technique of using ultrasound-locating means to generate the three-dimensional location data intravascularly. Based on recent advances in computer data analysis capability, the speed of analyzing the data obtained from a 3-D ultrasound locating system becomes feasible.
While an electrophysiology mapping and/or ablation procedure using an existing catheter has had promising results under x-ray imaging, reduction or elimination of x-ray exposure becomes a clinical need and a health issue to certain types of patients undergoing the catheter-based treatment. Therefore there is a need for an improved catheter system having intravascular ultrasound locating capabilities.